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Radioactive Fallout From Nuclear Weapons Testing

Radiation Facts
  • Immediately following an aboveground nuclear explosion, debris and soil can mix with radionuclides. This mixture is sent up into the air and then falls back to Earth. It is called fallout and it typically contains hundreds of different radionuclides.
  • Since the conclusion of the weapons testing in the 1980s, radionuclides in the atmosphere have largely decayed away.

Detonating nuclear weapons aboveground sends radioactive materials as high as 50 miles into the atmosphere. Large particles fall to the ground near the explosion site, but lighter particles and gases travel into the upper atmosphere. The particles that are swept up into the atmosphere and fall back down to Earth are called fallout. The highest particles can circulate around the world for years until they gradually fall to Earth or are brought back to the surface by precipitation. The path of the locations of the fallout depend on wind and weather patterns.

On this page:
  • About Radioactive Fallout From Nuclear Weapons Testing
  • Where to learn more


About Radioactive Fallout From Nuclear Weapons Testing

Fallout typically contains hundreds of different radionuclides. Some stay in the environment for a long time because they have long half-lives, like cesium-137, which has a half-life of  about 30 years. Most have very short half-lives, so decay away in a few minutes or a few days, for examples iodine-131, has a half-life of 8 days. Very little radioactivity from weapons testing in the 1950s and 1960s can still be detected in the environment now.

The United States conducted the first aboveground nuclear weapon test in southeastern New Mexico on July 16, 1945. Between 1945 and 1963, hundreds of aboveground test took place around the world. Over time the number and size (or yield) of these weapons increased, especially in the late 1950s and early 1960s. After the Limited Test Ban Treaty of 1963 was signed by the United States, the Soviet Union and Great Britain, most aboveground tests ceased. Some aboveground weapons testing by other countries continued until 1980. Since the end of aboveground nuclear weapons testing, the day-to-day radiation in air readings from monitoring sites has fallen. For many years, analysis of air samples has shown risk levels far below regulatory limits. In fact, results are now generally below levels that instruments can detect.

The EPA maintains a system of radiation monitors throughout the United States. These monitors were originally designed to detect radionuclides that were released after a nuclear weapon detonation. Now, the EPA uses this system, called RadNet, to look at background radiation levels at many locations across the United States. Background radiation is around us all the time, mostly from natural sources, like naturally occurring radon and uranium. For more information about the history of RadNet, please visit the Learn About RadNet webpage.

Some of the fallout radionuclides the EPA’s monitoring systems may detect include:

  • Americium-241
  • Cesium-137
  • Iodine-131
  • Strontium-90

Even though there is very little fallout that still exists in the environment, it is important to remember that recent fallout, within about 10 to 20 miles downwind of the detonation, can be very dangerous. This section talks about the different ways we can be exposed to radiation if a nuclear detonation occurs.

When a nuclear detonation occurs, people, plants, and animals can be exposed to the fallout in several ways. Livestock may eat contaminated plants or drink contaminated water. People who then eat this livestock will then still experience internal contamination, in which radioactive material ends up inside of our bodies, despite not consuming contaminated plants or water directly.

Radionuclides that are inhaled or ingested are not blocked by an external shield. These radionuclides interact with internal cells and tissues, which increases the risk of harmful health effects. When radionuclides can ingested, they can change the structure of cells, which is one of the ways people can develop cancer. The health risks from fallout have been described in many studies. One example is the Federal Radiation Council’s 1962 report, Health Implications of Fallout from Nuclear Weapons Testing through 1961. This is one of the reasons why radiation protection professionals work hard to protect people from unnecessary exposure to radiation.

The radioactive dust that settles on the environment around us is an example of potential external exposure. Radionuclides that emit alpha and beta particles would pose a lower external exposure threat because they do not travel very far in the atmosphere and are not as penetrating as more energetic radiation. Shielding, one of the three principles of radiation protection, prevents some external exposure because alpha particles are blocked by the dead skin cells that sit on the surface of our bodies. Gamma rays, however, travel much farther in the atmosphere, and are higher energy rays that can only be blocked by heavy shielding, like a concrete wall or a lead apron. These rays pose a higher external exposure risk.

This image shows alpha particles coming out of a source represented by blue and green balls.
Alpha particles come from the decay of the heaviest radioactive elements, such as uranium, radium and polonium.
This image shows beta emitters coming out of a source represented by blue and green balls
Beta-emitters are most hazardous when they are inhaled or swallowed.
This image shows gamma rays coming out of a source, represented by blue and green balls.
Gamma rays are often emitted along with alpha or beta particles during radioactive decay.
Where to Learn More

The U.S. State Department

The U.S. State Department negotiated three treaties that govern nuclear weapons testing.

The Treaty on The Limitation Of Underground Nuclear Weapon Tests
This treaty is also known as the Threshold Test Ban Treaty. It was signed in July of 1974 by the United States and the Union of Soviet Socialist Republics (USSR). It established a nuclear “threshold,” by prohibiting tests having a yield exceeding 150 kilotons (equal to 150 thousand tons of TNT).

Treaty Banning Nuclear Weapon Tests in The Atmosphere, in Outer Space and Underwater
The Test Ban Treaty of 1963 prohibits nuclear weapons tests “or any other nuclear explosion” in the atmosphere, in outer space and under water. It does not ban tests underground. The goal of the treaty is to end radioactive contamination of the environment.

The Comprehensive Nuclear Test-Ban Treaty (CTBT)
The CTBT is a legally binding global ban on nuclear explosive testing. The CTBT was opened for signature in 1996. The United States has signed the treaty, but it has not been ratified by the Senate.

The National Archives and Records Administration (NARA)

The National Archives is the U.S. Government’s collection of documents that records important events in American history. The NARA is the Government agency that preserves and maintains these materials and makes them available for research.

Teaching with Documents: Photographs and Pamphlet About Nuclear Fallout
This webpage contains a brief description of the nuclear arms race of the 1950s and 1960s. It also provides a 1950s pamphlet about fallout and several pictures related to nuclear weapons testing and fallout shelters.

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Last updated on May 1, 2025
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